The increasing interest in developing undersea technologies has required that scientists and engineers gather and process vast amounts of data. The undersea environment imposes limitations on a good many telemetry functions so that the validity of the gathered data often is compromised.
One earlier communication system is disclosed in U.S. Pat. No. 2,798,902 by D. R. Kursman et al. Signals are picked up by a microphone, are converted to electrical signals and fed through a switch and through a balanced modulator which is also coupled to an oscillator. The upper and lower sidebands are transmitted from the transducer and, in like manner, received by the transducer. The Kursman et al reference provide only for the oscillator, for the asynchronous demodulation of an incoming signal and does not concern itself with synchronous as well as asynchronous operation.
A later development is disclosed by J. T. Kroenert in his U.S. Pat. No. 3,164,800. Here again, the bidirectional transmission through a water medium relies upon asynchronous system operation and, as such, the Kroenert telephony system stands as a noteworthy advance in the state of the art.
The diver communication system of U.S. Pat. No. 3,789,353 employs a transmitter and a double channel speech signal receiver. The adjustable gain amplifiers included in the channels have the usual balanced demodulator and interconnected low pass filter. While being a step forward in the state of the art, the emphasis of this system appears to be directed to signal enhancement that addresses the problems attendant diver communications.
A later development by John Weston in U.S. Pat. No. 4,039,999 employs ultrasonic frequency signals modulated by an audio input that particularly lend themselves to a helmet-mounted system. The thrust of the Weston system is to avoid the problems normally associated with crosstalk or scrambling among a number of divers simultaneously communicating within a given area. The capability for asynchronous or synchronous demodulation is not provided for.
Thus, there is a continuing need in the state of the art for an undersea transmitting and receiving system having the capability for synchronous and asynchronous operation that assures both long range communications and greater reliability, particularly when a number of channels are being simultaneously modulated and demodulated.